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Physical, morphological and strength properties of Jana Manjung coal ash mixture for geotechnical applications.


Malaysia has a coal mining history dating back as far as 1851. The country promotes coal as an option for fuel in electric power generation. Increasing of electricity needs however, has led to increase the coal consumption and thus significantly produced more coal waste product. After burning process, the coal residues by-products contain approximately 80% of fly ash and 20% of bottom ash. The chemical composition give color ranges from gray to very dark of both type of waste. Despite its utilisation application, landfill has been the primary method of disposal of these waste materials [1]. Additionally, the ash waste contains contaminants like mercury, cadmium and arsenic which can cause serious health problems. Therefore, growing volume of fly ash and bottom ash waste posed health, environmental and economic impact.

Previous researches have proved the individual properties of fly ash and bottom ash can be used as in most construction such as road construction, embankment fill, back fill, soil improvement, and concrete admixture for cement. However, there is very limited study related to fly ash-bottom ash mixtures and very few information about the performance of using the mixtures for the construction or geotechnical work. Hence, this study proposes the mix of both fly ash and bottom ash as new materials that can be used in geotechnical application such as embankment construction, roadbase, and soil stabilization.

The specific objectives of this study are to (i) determine the chemical and physical coal ash mixture (ii) study the morphological and mineralogical of different proportion of coal ash mixtures using SEM and XRD (iii) study the compaction behaviour and shear strength properties of compacted fly ash and bottom ash mixture with curing time.


The fly ash and bottom ash sample obtained from Jana Manjung Power Plant, Perak. The mix proportions in percent (BA100, FA25 BA75, FA50 BA50, FA75 BA25, FA100) were measured by weight of total mixtures. Both morphological and mineralogical analyses were carried out by Scanning Electron Microscope (JEOL, JSM 6460 LA) and X-ray powder diffraction (Bruker, D2 Phaser). The unconfined compression test (UCT) was carried out based on the compacted sample with moisture content at 95% maximum dry density.


Physical Properties and pH:

The results show that as fly ash content increase, the particle size distribution is becoming well-graded (Fig. 1).


The composition of fine particles is increasing from the mixtures of which contain more fly ash. Particle size distribution of fly ash and bottom ash mixtures covers the range between clay to gravel with the size ranging from 0.001 mm to 5.0 mm. The mixture which contains only bottom ash (0% FA) can be classified as coarse-grained soil particle which has distributed at range of 0.08 mm to 5.0 mm.

The [G.sub.s] results for Jana Manjung coal ash mixture are shown in Table 1. Value of [G.sub.s] decreases as the content of fly increased in the mixture. The difference in range of [G.sub.s] can be attributed to two which is the chemical composition and presence of hollow fly ash particles or either bottom ash particles with porous or vesicular structure [2, 3].

Results show that [G.sub.s] of bottom ash are higher than [G.sub.s] of fly ash. Composition of fly ash and bottom ash at different rates show that [G.sub.s] of the mixture depend on the particle size of the grain in mixture. [G.sub.s] value of fly ash is lower than that G of the bottom ash which might be attributed to the higher contents of iron oxide and the larger size of the bottom ash particles. Results of pH value show that composition of 100% FA has higher value of pH compared to bottom fly ash (Table 1). pH value increases as the fly ash increase in the mixture. The fly ash contains higher free lime and alkaline oxides exhibits higher pH values [2].

Morphological and Mineralogical:

Morphologies of the coal ash mixtures from Jana Manjung are presented in Fig. 2 at curing day 7 and day 28.


Results of SEM show that for BA 100, the particles were angular and irregular in shape and had rough and uneven surface texture for day 7 (Fig. 2 (a)). This is because bottom ash contains more unburnt carbon than fly ash. The fine fractions of shattered and irregular bottom ash particles were predominant in the compacted specimens for the 28 days (Fig. 2 (b)). The particle surface also shinny, clean and free of dust, which tends to be relatively inert due to the bigger size and fused that attributes to less pozzolanic mechanism.

However, as a fly ash composition increased from 0% to 100%, the angular and irregular shapes replaced by rounded and spherical in particles shape. A distinct morphological difference of two curing period was extent of the agglomeration of particles, which appeared to be more prevalent at 28 day curing period (Figs. 2 (c) and (d)). At this period, the number of irregular shaped particles is increased as well as particle size, which due to high pozzolanic reaction. The morphology fly ash is well rounded with thin wall, glassy spheres and their surface appear to be very smooth after 28 day curing period (Fig. 2 (f)). Both of fly ash and bottom ash have particle agglomerations which ranged from lightly cemented to strongly bonded [3]. Particle morphology and particles particle size of a mixture play a vital role in determining its engineering behaviour [1]. The morphological characteristics of fly ash and bottom ash affected their specific gravity, particle strength, and consequently other mechanical properties to varying degrees.

Results of XRD analysis of coal ash mixtures with curing time of day 7 and day 28 is shown in Table 2.

For both curing period (7 day and 28 day) almost similar mineral contents were detected except for additional compound of calcium aluminium silicide, silicon carbide, calcium, and moissanite at range of 5.5% to 24%.

Engineering Properties of Coal Ash Mixture:

Compressibility curves for compaction test are shown in Fig. 3. Maximum dry density and optimum moisture content for each coal ash mixtures with different proportion of fly ash and bottom ash is summarized in Table 3.


Through the mixing process of both materials, it is shown that the optimum moisture content fluctuated as the fly ash content increased (Fig. 3). Since the physical properties of bottom ash are almost identical to sand, it required higher moisture to be compacted to obtain the maximum dry density. Inconsistency of the values may be primarily due to the larger differences of its specific gravity and the cause due to crushing of bottom ash during the compaction.

Table 3 shows the value of undrained shear strength of each mixture at curing period of 7 day and 28 day. Data were obtained from the plotting graph of stress versus strain as shown in Figs. 4 (a) and (b). Unconfined compressive strength (qu) obtained from peak of the value for each trend, which then calculated by the following


[S.sub.u] = [q.sub.u] /2 (1)


It was found that 50% of fly ash composition had the highest shear strength compared to the other mixture. This may be due to the optimum bonded between fly ash and bottom ash particles. In addition, the shear strength of coal ash mixtures increased with the increase in curing period which might be due to the pozzolanic reactions and also some agglomerates bonded particles formed due to the crystal growth. In morphology term, as curing time is increased the size of coal ash mixtures is increased and help to provide more strength between particles of coal ash mixtures (Figs. 2 (d) and (f)).

Value of undrained strength at failure of the mixture containing of 100% fly ash is 309.23 kPa. At this point, particles inside mixture at appropriate moisture content in the sample make both material bonds very well. This interlocking created from the chemical reaction as fly ash gain some shear strength from the hydrolysis process when the particle reacted with water [4]. On the other hand, bottom ash does not have any cementitious reactions when subjected to moisture and it reacts the as sand which is lack in shear strength. The obtained undrained value for 100% bottom ash is 10.72 kPa after 28 day cured, because the bottom ash exhibits small cohesive characteristics (Table 3).


Tests were performed on fly ash composed of fine nearly spherical particles ranging mostly in size from 0.001 mm to 1.00 mm, bottom ash composed of coarse particle ranging in size sand to small gravel and mixed into two. Morphological characteristics of the mixtures at 28 days curing period affected the specific gravity, particle size and particle strength due to pozzolanic reactions and bonded particles of crystal growth. Dry density recorded between 1.46 kN/[m.sup.3] to 4.27 kNm[/.sup.3] for various mixtures of fly ash and bottom ash. The undrained shear strength at 28 cured day were observed at 10.72 kPa to 309.23 kPa. Samples of compacted ash mixtures (e.g. 50% fly ash) at higher compaction level exhibited even higher shear strength.


Article history:

Received 25 September 2014

Received in revised form 26 October 2014

Accepted 25 November 2014

Available online 31 December 2014


[1] Mahlaba, J.S., E.P. Kearsley, R.A. Kruger, Physical, 2011. chemical and mineralogical characterization of hydraulically disposed fine coal ash from SAsOl Synfuels, Fuel., 90: 2491-2500.

[2] Pandian, N.S., 2004. Fly ash characterization with reference to geotechnical applications, J. Indian Inst. Sci., pp: 189-216.

[3] Kim, B., M. Prezzi, R. Salgado, 2005. Geotechnical properties of fly ash and bottom ash mixtures for use in highway embankments, J. Geotech. Geoenviron. Eng., 131: 914-924.

[4] Muhardi, A., K.A. Marto, A.M. Kassim, F.W. Maktar, S.L. Lee, 2010. Yap, Engineering Characteristics of Tanjung Bin Coal Ash, Elec. J. Geotech. Eng., 15: 1117-1129.

(1) Afizah Ayob, (2) Mohd Zulham Affandi Mohd Zahid, (1) Mohd Faiz Mohammad Zaki, (1) Siti Hasmah A. Hamid, (1) Mohd Al-Hafiz Mohammad Yussuf, (1) Ashraf Nasuha Mohd Yunus

(1) School of Environmental Engineering, Department of Civil Engineering, Kompleks Pengajiaan Jejawi 3, Universiti Malaysia Perlis, Arau 02600, Perlis, Malaysia

(2) Department of Civil Engineering Technology, Faculty of Engineering Technology, Universiti Malaysia Perlis, Pauh Putra, 02600 Arau, Perlis, Malaysia

Corresponding Author: Afizah Ayob, School of Environmental Engineering, Department of Civil Engineering, Kompleks Pengajiaan Jejawi 3, Universiti Malaysia Perlis, Arau 02600, Perlis, Malaysia

Table 1: Specific gravity and pH value of coal ash mixture

Composition (%)   Specific gravity, [G.sub.s]   pH

BA 100            2.54                          9.7
FA 25 BA75        2.41                          10.1
FA 50 BA 50       2.36                          10.3
FA 75 BA 25       2.25                          10.4
FA 100            2.22                          10.6

Table 2: XRD analysis of coal ash mixture at different curing period

Coal ash                                         Curing period
composition (%)   Crystalline composition (%)    7 day    28 day

BA 100            Berlinite                      76. 3    --
                  Coesite                        23.7     63.1
                  Calcium Aluminum Silicide      --       17.2
                  Quarts                         --       14.8
FA 25 BA 75       Berlinite                      --       7.8
                  Coesite                        73.9     78.8
                  Calcium Sodium Sulphate (IV)   22.8     --
                  Nickel                         3.3      --
FA 50 BA 50       Coesite                        97.3     78.71
                  Silicon carbide                --       13.4
                  Quartz                         2.7      21.3
FA 75 BA 25       Coesite                        0.7      52.0
                  Calcium                        --       24.7
                  Quartz                         99.3     23.3
FA 100            Calcite                        20.0     21.8
                  Graphite                       60.3     13.0
                  Quartz                         19.7     59.7
                  Moissanite                     --       5.5

Table 3: Maximum dry density with its moisture content of compaction
test and shear strength at 7 day and 28 day curing period

Fly ash           Maximum dry              Optimum moisture
composition (%)   density (kN/[m.sup.3])   content (%)

BA 100            1.46                     19.21
FA 25 BA 75       1.54                     19.84
FA 50 BA 50       4.27                     15.23
FA 75 BA 25       5.32                     21.01
FA 100            1.86                     33.72

Fly ash           Curing period
composition (%)   7 day                       28 day
                  Undrained shear             Undrained shear
                  strength, [S.sub.u] (kPa)   strength, Su (kPa)

BA 100            7.74                        10.72
FA 25 BA 75       11.71                       146.40
FA 50 BA 50       17.86                       310.11
FA 75 BA 25       34.28                       289.04
FA 100            36.66                       309.23
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Article Details
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Author:Ayob, Afizah; Zahid, Mohd Zulham Affandi Mohd; Zaki, Mohd Faiz Mohammad; Hamid, Siti Hasmah A.; Yuss
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:9MALA
Date:Nov 1, 2014
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